In the manufacture of a semiconductor device such as a large scale integrated circuit (LSI) and a very large scale integrated circuit (VLSI), or a liquid crystal display panel, a pattern is made by irradiating a light to a semiconductor wafer or an original plate for liquid crystal, but if a dust is attached a photomask or a reticle (hereinafter collectively referred to as “photomask” for simplicity) which is used during the irradiation operation, the dust causes the pattern to have roughened edges or black stains on a base, and thus leads to problems such as damaged dimensions, poor quality, and deformed external appearance.
In order to prevent these problems, the operation of exposing the wafer or substrate is generally conducted in a clean room. However, even so, it is not always easy to keep the photomask dust-free, and hence in order to fend off the dust from the surface of the photomask, a pellicle is attached to cover the exposure photomask. In this manner, the dust is prevented from reaching the surface of the photomask but can only alight on the pellicle membrane so that, if the exposure light is set to focus on the pattern of the photomask, the dust on the pellicle membrane fails to shadow itself in the transferred pattern.
In general, a pellicle is made by tensely adhering a transparent pellicle film made of a highly light transmitting material such as cellulose nitrate, cellulose acetate, fluorine-containing polymer and the like to one of the two annular frame faces of a pellicle frame made of aluminum, stainless steel, polyethylene or the like, using as the glue either a solvent capable of dissolving the pellicle film, which is applied to said annular frame face (hereinafter this face is called “upper frame face”) and then air-dried before receiving the film (ref. IP Publication 1), or an adhesive such as acrylic resin, epoxy resin or the like (ref. IP Publication 2). The other annular frame face (hereinafter called “lower frame face”) of the pellicle frame is paved with an agglutinant made of polybutene resin, polyvinyl acetate resin, acrylic resin, silicone resin or the like for attaching the pellicle frame to a photomask, and over this agglutinant layer is laid a release liner (tape) to protect the agglutinant layer.
This pellicle is glued to a photomask (a substrate made of a transparent material such as glass) via the agglutiant layer so as to prevent a foreign particle from adhering to the pattern on the photomask; however, if the agglutinant layer is not completely laid on the photomask but is broken to allow a formation of a leak passage, then a foreign particle could enter the inner space of the pellicle through the leak passage and could affect the pattern on the photomask.
In order to secure that the agglutinant layer of the pellicle is leaklessly bonded to the photomask, one can increase the bonding pressure and/or the length of the pressure application time. But in recent years, the tendency has been such that, when the pellicle is being bonded to the photomask, the bonding pressure is kept relatively low so as to prevent the pattern on the photomask from undergoing deformation, and that the pressure application time is decreased so as to shorten the manufacturing time. For example, IP Publication 3 describes a pellicle for lithography wherein the flatness of the agglutinant layer is heightened and its adhesive strength is reduced to a range of 1 N/m-100 N/m so as to prevent the photomask from deforming.